1. Field of the Invention
The present invention relates to metal corrosion inhibitors, inhibited acid compositions and methods of using the acid compositions in the treatment of hydrocarbon producing wells.
2. Description of the Prior Art
Aqueous acid solutions are often utilized to treat subterranean hydrocarbon producing formations penetrated by well bores whereby the production of hydrocarbons therefrom is increased. One such production stimulation treatment generally referred to as "acidizing" involves the introduction of an aqueous acid solution into a subterranean formation under pressure so that the acid solution flows through the pore spaces of the formation. The acid reacts with acid soluble materials contained in the formation thereby increasing the size of the pore spaces and increasing the permeability of the formation. Another similar treatment known as "fracture-acidizing" involves the formation of one or more fractures in the formation and the introduction of an aqueous acid solution into the fractures to etch the fracture faces whereby channels are formed therein when the fractures close. The acid also enlarges the pore spaces in the fracture faces and in the formation.
The above described acidizing and fracture-acidizing well stimulation treatments have been performed successfully for many years. However, a continuous problem which accompanies the treatments is the corrosion of metal surfaces in pumps, tubular goods and equipment used to introduce the aqueous acid solutions into the subterranean formations to be treated. The expense associated with repairing or replacing corrosion damaged tubular goods and equipment can be very high. The corrosion of tubular goods and downhole equipment is increased by the elevated temperatures encountered in deep formations, and the corrosion results in at least the partial neutralization of the acid before it reacts with acid-soluble materials in the formations.
Another problem encountered in acidizing and fracture-acidizing hydrocarbon producing formations involves the precipitation of undesirable compounds which can damage the producing formations by filling and plugging the pore spaces therein. For example, the acid solution may dissolve iron-containing scale from pipe and equipment and iron-containing minerals present in the formation. As the acid reacts and the pH of the acid solution increases, dissolved iron present in the solution in the form of ferric ion precipitates as ferric hydroxide. The ferric hydroxide precipitate can plug the formation and thus cause serious well damage.
When the well being treated is sour, i.e., the hydrocarbon fluids produced contain a significant quantity of sulfide compounds, and particularly hydrogen sulfide, problems are often encountered as a result of the precipitation of iron sulfide from the acid treating solution. Sulfide ions reduce ferric ions to ferrous ions with the result that the precipitation of ferric hydroxide is reduced. However, elemental or free sulfur is produced in the reduction reaction which precipitates. Sulfur precipitate can also plug the formation and cause serious damage thereto.
In addition, sulfide ions react with ferrous ions to form ferrous sulfide which also can precipitate. The ferrous sulfide precipitates from an acid solution when the pH of the solution reaches a level greater than about 1.9. Because the pH level of a spent acid solution used to treat subterranean well formations is greater than 1.9, ferrous sulfide dissolved in an unspent acid solution precipitates before the acid solution becomes spent. The ferrous sulfide precipitate can also plug the treated formation and cause serious damage to the well.
While various hydrogen sulfide scavenger compositions have been developed and used heretofore to react with and tie up sulfide ion in acid solutions, such hydrogen sulfide scavengers are often ineffective at low temperatures and/or are highly toxic.
Certain crude oils contained in subterranean formations precipitate sludge upon contact with aqueous acid solutions during acidizing treatments. The precipitates formed are mainly asphaltenes, resins, paraffins and other high molecular weight hydrocarbons which, once formed, are very difficult to remove from producing formations.
Methods of preventing or reducing sludge formation upon contact between sludging oil and an aqueous acid solution have been developed and used heretofore. The methods have included combining anti-sludging agents and other additives with the aqueous acid solution whereby the precipitation of sludge is reduced when the resulting acid mixture contacts the sludging oil. While the methods and anti-sludging agents have achieved varying degrees of success, the anti-sludging agents and other additives included in the acid mixtures have often separated even though dispersants have been included therein. When the resulting non-homogeneous acid mixtures have contacted sludging oil in subterranean formations, separated portions of the acid mixtures have still caused the precipitation of sludge in the formations.
Thus, there are continuing needs for improved corrosion inhibitors, improved inhibited acid compositions and improved methods of acid treating subterranean producing formations whereby greater and more reliable corrosion inhibition of metal surfaces is provided; ferric ion dissolved in the acid compositions is reduced to ferrous ion whereby ferric hydroxide precipitates and the like are prevented; sulfide ions dissolved in the acid are prevented from reacting with iron ions whereby free sulfur or ferrous sulfide precipitates and the like are prevented; and anti-sludging agents mixed with the acid compositions are prevented from separating and allowing the precipitation of sludge.